DESCRIPTION: (Adapted from the application) Prolonged bed rest or weightlessness in humans results in a number of adverse cardiovascular consequences, often referred to as cardiovascular deconditioning. Prominent among these are orthostatic intolerance and decreased exercise capacity. Rat hindlimb unweighting is an animal model of cardiovascular deconditioning, and results in similar cardiovascular consequences. Cardiovascular adjustments to both orthostatic challenges and exercise require the integrated response of cardiovascular reflexes and the appropriate vascular response to reflex stimuli. Therefore, the orthostatic intolerance and decreased exercise capacity associated with cardiovascular deconditioning may be due to reflex dysfunction, vascular dysfunction, or both. The proposed studies will test the general hypothesis that hindlimb unweighting in rats results in impaired reflex control of the vasculature. The investigators postulate that arterial and cardiopulmonary baroreflex control of the sympathetic nervous system is impaired in conscious hindlimb unweighted rats. They further propose that the ability of the vasculature to respond to sympathomimetic vasoactive stimuli is compromised. There are 4 specific aims: 1) To evaluate arterial and cardiopulmonary baroreflex regulation of renal and lumbar sympathetic nerve activity in conscious rats subjected to 14 days of hindlimb unweighting, 2) To examine the interaction between arterial and cardiac baroreflex control of sympathetic nerve activity in conscious hindlimb unweighted rats, 3) To evaluate changes in afferent and/or central nervous system mechanisms in baroreflex regulation of the sympathetic nervous system, and 4) To evaluate changes in vasomotor reactivity of visceral and skeletal muscle vascular beds in conscious rats after 14 days of hindlimb unweighting.